The spermatophores of Aculus cornutus (Banks), Phyllocoptruta oleivora (Ashmead), Eriophycs insidiosus Keifer & Wilson, E. pyri (Pagenstacher), Diptacus gigantorhynchus (Nalepa), and Novophytoptus sp. (undescribed) representing all 3 eriophyoid families consist of an enlarged base, a stalk, an expanded apical head, and a sac, presumably for sperm. Virgin female A. cornutus and P. oleivora produced only males when they were isolated without access to spermatophores but produced both males and females when they were exposed to spermatophores. Female A. cornutus produced several females after they were inseminated from 1 spermatophore each, even though they left the base, stalk, and at least part of the head of the spermatophore attached to the leaf. Male A. cornutus and P. oleivora produced from 16 to 30 spermatophores per day and male A. cornutus deposited spermatophores within 24 hours after emergence. Preliminary evidence indicated that sperm of A. cornutus remains viable in spermatophores on the leaf for 3 days and in the female for 4 or more days.